Asp.net Core Post parameter is always null

I just had to correct the double quotes in your POST request and it worked. Try this:

{"Ean":"1122u88991","Name":"Post test","Description":"Post test desc"}

See screenshot below.

I experience a similar problem but I had no check for the validity of the ModelState as the OP. When this happens, inspecting it in debug mode will likely point you to what's wrong with the model:

In this example I was using an invalid string 'A' as test value for a Guid property, so the model was always null.